The SIT board approved leave this week, said Mrs Simmonds, who has been selected by the National Party to contest the Invercargill electorate at the 2020 general election.
She will take leave from June 29, until the election, on September 19.
"I will be completely away from SIT and will be concentrating entirely on the campaign.
"After the election, if I’m elected as the member of Parliament for Invercargill, then I have to vacate my position at SIT. If I’m not elected, then I start back my work at SIT from Monday, September 21."
Deputy chief executive Maree Howden will be acting chief executive while Mrs Simmonds is away.
Mrs Simmonds has been in the role for 23 years.
While rumours of her candidacy had been around for about 15 years, she said she decided to consider it when incumbent MP Sarah Dowie announced her retirement in February.
Concerns for the future of SIT and for sectors feeling the pinch, including agriculture, prompted her to stand this year.
"I think it is appropriate to have this separation to not have any conflict of interest."
